1. Field of the Invention
This invention relates to a counterbalance valve which controls the supply of fluid to and from a fluid operated motor which raises, lowers and supports a load.
2. Description of the Prior Art
In its simplest form, the function of a counterbalance valve can be achieved by a circuit which incorporates a relief valve and a check valve which are connected in parallel to the load raising side of a fluid operated motor. The relief valve is set somewhat above the maximum desired system pressure. The check valve permits the free flow of pressure fluid to the fluid motor when the load is lifted, but closes when the load is stationary or lowered. To lower the load, fluid is supplied to the load lowering side of the motor to reverse the motor so that the sum of the force of the load and the force of the fluid acting to reverse the motor exceeds the setting of the relief valve and the load is lowered. A problem with using a relief valve in parallel with a check valve is that, if there is no load, fluid at a pressure above the setting of the relief valve must be supplied to drive the motor in reverse. This represents a great deal of wasted energy.
One way in which to lessen the energy lost in operating an unloaded fluid motor controlled by a relief valve in parallel with a check valve is to use a pilot operated relief valve which has an integral check valve. Such a valve has an adjustable spring which acts on one end, which has a small area, of a two-area spool to set the maximum system pressure. The load raising side of the motor is connected to the opposite end of the spool and the pressure fluid acts on the same area to oppose the spring. When the load is raised, fluid flows past the check valve to operate the fluid motor. When the load is lowered, a portion of the fluid in the load lowering side of the motor is directed to a pilot port connected to a larger area on the spool which acts in conjunction with the load pressure on the small area to move the spool against the spring to release the fluid from the opposite side of the motor. If there is no load on the motor, the amount of fluid pressure required to open the valve to drive the fluid motor in the load lowering direction is the maximum system pressure divided by the ratio of the spool areas.
If the ratio of the areas is 3:1, the pressure of the fluid directed to the pilot port must be one-third of the maximum system pressure set by the spring. This still represents a great deal of lost energy.
The aforementioned pilot operated relief valve is normally controlled by a four-way, open-center (all ports to tank) type valve with a throttling spool. To raise a load the valve port connected to the load lowering side of the motor and the pilot stage of the valve is open to tank and the valve port connected to the load raising side of the motor is open to pressure. To lower the load, the port connections are reversed.
A problem arises if it is desired to lower the load slowly. In order to lower a load slowly with an open-center throttling spool, the lever is moved such that pressure is supplied simultaneously to the load lowering side of the motor and to tank. The proportion of fluid to the load lowering side is gradually increased until the pressure fluid in the pilot stage is sufficient to move the spool and the motor can operate. If the load falls too fast, the pressure in the pilot stage falls, and the setting of the counterbalance valve is raised which stops the load from lowering. Since a pressure controlled counterbalance valve is a fast response valve, the valve starts and stops rapidly with the result that severe shocks are imposed on the hydraulic system, particularly when a heavy load is being lowered. Consequently, it can be seen that attempting to simultaneously control the pressures to a pilot operated relief valve by a four-way, open-center type valve is difficult and results in an unsatisfactory operation of the relief valve when a heavy load is being lowered.
One type of counterbalance valve which overcomes some of the problems mentioned above which occur when a heavy load is being lowered is shown in U.S. patent application Ser. No. 3,165, which is assigned to the assignee of the instant invention. The valve is used in a system in which a motor is driven in one direction to raise the load and in another direction to lower the load. Flow of fluid to the motor from a pump is controlled by a four-way valve. The counterbalance valve has a pilot operated metering spool with a variable orifice which passes fluid from the motor when a load is lowered and a pressure compensator piston which maintains a fixed pressure differential across the orifice to maintain a fixed fluid flow through the metering orifice for a set metering spool position. This arrangement maintains a constant rate of descent for a heavy load.
Although the counterbalance valve shown in Application Ser. No. 3,165 is a significant improvement over existing counterbalance valves, the valve is unable to prevent some jerkiness during changes in the pressure of the pilot fluid. This jerkiness occurs because the valve tries to respond instantly to each change in pressure of the pilot fluid which results in a loss of stiffness in the system.
It is desirable to have a pilot-operated counterbalance valve which is relatively insensitive to minor changes in pilot pressure which results in a stiffer system.